Subwooferin'...newbee seeks advice.

I recently acquired a pair of Apogee Slant 6's (6.5" Vifa woofer crosses over to a 26" ribbon at 1150 Hz) that I placed in my 22.5' x 24.5' room. I'm quite happy with the overall sound except, as you would guess, for bass reach/limitations. While the little woofers sound pretty good within their limits there are some CDs in my collection which overstress them. I'm investigating which subwoofers might work best with them and am thinking something like a powered pair with high pass filters built in, perhaps using 10" or 12" drivers, would be quick enough and would keep the very lowest frequencies out of the Apogees.

I'm using a Plinius 8150 integrated amp which has identical pairs of speaker outputs and also the "preamp out" feature.

Can anyone recommend a subwoofer with transparent high pass filters that won't affect, or will have very little affect, on the qualities of the main speakers? Thank you.